Stabilized alpha-cyanoacrylate compositions

ABSTRACT

The disclosed compounds are adducts of imidazole or derivatives thereof and sulfur dioxide. The adducts are apparently of the Lewis acid-Lewis base type, and are useful as latent curing agents for epoxide resins. The adducts are prepared by interacting the imidazole and sulfur dioxide under anhydrous conditions. Equimolar imidazole-SO2 adducts can serve as a source of sulfur dioxide at room temperature.

This application is a division of copending application Ser. No. 445,862filed Feb. 26, 1974, now U.S. Pat. No. 3,943,146, which in turn is adivision of application Ser. No. 262,557 filed June 14, 1972 and nowU.S. Pat. No. 3,839,282.

FIELD OF THE INVENTION

This invention relates to sulfur dioxide-imidazole adducts and methodsfor making them, the "imidazole" portion of the adduct being eitherimidazole itself (C₃ H₄ N₂) or a derivative thereof. An aspect of thisinvention relates to latent epoxy curing agents and latent curableepoxide resin systems containing sulfur dioxide-imidazole adducts. Afurther aspect of this invention relates to a source for an acidstabilizer in a closed system.

DESCRIPTION OF THE PRIOR ART

It is well known that sulfur dioxide is a Lewis acid, though not nearlyso strong a Lewis acid as, for example, boron trifluoride. It is alsoknown that nitrogen bases (e.g. amines) are Lewis bases and can reactwith sulfur dioxide to form adducts. The literature relating to suchadducts is extensive, representative examples being W. C. Fernelius,Ed., Inorganic Synthesis, II, McGraw-Hill, N.Y. (1946); W. E. Byrd,Inorganic Chemistry 1, p. 762 (1962); K. R. Hoffman et al, J. Am. Chem.Soc. 68, p. 997 (1946), H. A. Hoffman et al, J. Am. Chem. Soc. 70, p.262 (1948). The Byrd article indicates that the exact structure ofaromatic amine-sulfur dioxide complexes is not well understood, since itis possible that the sulfur dioxide could be bound to the adduct via thepi-complex of the aromatic ring. The picture is further complicated bydata in the Byrd article and both Hoffman et al articles indicating thatsome of these aromatic (or heterocyclic-aromatic) amine-sulfur dioxideadducts or complexes are unstable, though there is little doubt thattrue adducts, rather than simple mixtures, are formed.

According to U.S. Pat. No. 2,270,490 (Wood), issued Jan. 1942,morpholine and sulfur dioxide react to form a compound useful as aphotographic developer, local anesthetic, or antioxidant, but in thiscase it is suggested that the compound can be a true salt, i.e. a saltof the cation-anion type. Some of the aromatic amine-sulfur dioxideadducts exhibit crystalline character and have sharp melting points, butprobably lack this high degree of ionic character. The nature of theamine (aliphatic, aromatic, aromatic heterocyclic, nonaromaticheterocyclic, etc.) appears to have significant effects upon the natureof the sulfur dioxide adduct or compound, but these effects have notbeen explored fully enough to postulate any general rules for all thepossible adducts.

Apparently none of these prior art adducts or compounds has beeninvestigated for use as a latent catalyst or initiator or curvature inepoxy resin technology, though amines per se have been investigatedextensively. Several different Lewis acid-Lewis base adducts of theamine-boron trifluoride type have been carefully studied by epoxy resinchemists, and various theories have been proposed to explain theirpotency or latency, as the case may be, in curing diglycidylether-bisphenol A epoxides at various cure temperatures. See, forexample, Harris et al, J. Appl. Polymer Science 10, p. 523 (1966). TheHarris et al article provides little or no guidance for one attemptingto use a Lewis acid-Lewis base adduct derived from sulfur dioxideinstead of boron trifluoride. The degree of latency of an amine -- BF₃adduct appears to be independent of its stability (Harris et al, opcit., pp. 523-525 and 527), but the stability of amine -- SO₂ adducts isso variable that latency might very well be a function of the stabilityof the adduct.

Imidazole and its derivatives have been used as non-latent initiators orcurative-catalysts for epoxide resins, and various approaches have beenused to make latent compounds or complexes containing imidazole nuclei;see, for example, U.S. Pat. No. 3,553,166 (Anderson et al), issued Jan.5, 1971. Apparently, imidazole-sulfur dioxide adducts have never beenreported in the literature and have never been proposed for use as epoxycuring agents or for any other purpose. In view of the variations in thestability of different types of amine-sulfur dioxide adducts and thelack of guidance in the prior art as to their behavior as epoxyinitiators, it would be difficult at best to predict the stability andutility of an imidazole-sulfur dioxide adduct. Nor is it even possibleto predict with certainty from the available prior art if such an adductcan even be made and what its structure might be. Analogies betweenimidazole and other heterocyclic and/or aromatic and/or aliphatic aminesare difficult to draw, due to the uniqueness of the imidazole nucleus##STR1##

Accordingly, this invention contemplates synthesizing adducts fromimidazole or derivatives thereof and sulfur dioxide. This inventionfurther contemplates using these adducts in polymer chemistry andformulating either curable latent epoxy resin systems or stabilizedacrylate monomers containing the adducts.

SUMMARY OF THE INVENTION

Briefly summarized, the present invention involves the synthesis ofimidazole-sulfur dioxide adducts and the discovery that these adductsare useful both as a source for an acidic stabilizer and in theformulation of latent curable epoxides. These latent curable systemshave a shelf life of weeks or months (e.g. at least 6 months) and arerendered non-latent at elevated temperatures (e.g. above 50° C.,preferably above 130° C.); that is, the typical epoxide curing reactionscan be made to occur expeditiously at these elevated temperatures. Thecompounds of this invention can be prepared by exposing imidazole or aderivative thereof (optionally dissolved in a solvent) to liquid orgaseous sulfur dioxide, or a saturated solution thereof, under anhydrousconditions for a period of several minutes until the compoundprecipitates out as a solid sulfur dioxide-imidazole adduct. Theapparent behavior of these solid products in the presence of curableepoxy resins is evidence not only of utility, but also of a stable,adduct-like structure, as opposed to a mere physical mixture. Theadducts decompose upon heating into sulfur dioxide and imidazole (or animidazole derivative) and thereby serve as a latent source for eitherthe imidazole or sulfur dioxide.

DETAILED DESCRIPTION

Adduct-like compounds made according to the present invention can berepresented by the following formula:

    (Imid).sub.n.SO.sub.2                                      (I)

wherein Imid is imidazole or a derivative thereof, i.e. a nucleus of theformula ##STR2## wherein R¹, R², R⁴, and R⁵ represent substituentsselected from the group consisting of hydrogen, an aliphatic radical,and an aromatic radical, and, together R⁴ and R⁵ can be the residue of afused ring.

The n term represents a small number less than 4, preferably an integer.The exact structure of the various species encompassed by n is not aknown, but available evidence substantiates the formation of Lewisacid-Lewis base adducts wherein n=1, 2, or 3 or mixtures of thesespecies. When n=1, these compounds have a tendency to lose sulfurdioxide slowly until the n=2 species (the most stable) is formed.

The terms R¹, R², R⁴, and R⁵ represent suitable substituents orhydrogen, the preferred substituents being non-hindering organicradicals such as alkyl or aryl radicals or, particularly in the case ofR⁴ and R⁵, fused rings. For convenience, the term "an imidazole" or "animidazole nucleus" is used in this specification to denote bothimidazole itself (C₃ N₂ H₄) and the imidazole derivatives of formula(II), set forth previously.

The term curing agent is used to denote agents which assist orparticipate in hardening or crosslinking or polymerization reactionswhich solidify or increase the viscosity of liquid epoxide monomers orprepolymers or convert solid epoxides to tough, durable thermosetmaterials. Curing agents are referred to as "hardeners" or"crosslinkers" in some contexts, because of their ability to converteven the liquid monomers or prepolymers to thermoset solids. It is alsocommon in the art to refer to imidazole as a catalyst or initiator sinceit assists in the opening of the oxirane ring. However, it isestablished that imidazole can make a contribution to the properties ofthe cured epoxide and is thus more than a simple catalyst. Forconsistency of terminology, the term curing agent is used in thisspecification.

In the art of curing or hardening epoxide resins, a latent curing agentis one which is effective only under certain specific conditions, e.g.temperatures above 50° or 100° C. A latent curing agent can therefor beincluded in a one-part curable system which is storable for long periodsof time at normal ambient temperatures, and the storable system can besaid to have good storage ability or a long shelf life or pot life. Theshelf life of a liquid one-part system can be conveniently determined byobserving its viscosity; any tendency toward premature gelation will, ofcourse, be evidenced by an increase in viscosity.

Adducts of an imidazole and sulfur dioxide can be synthesized by one ofthe following methods, provided that anhydrous conditions aremaintained.

1. An imidazole is dissolved in a solvent such as acetonitrile andgaseous sulfur dioxide is bubbled through the solution. An exothermicreaction occurs. Upon dilution with the solvent, a white precipitateforms which is the imidazole-sulfur dioxide adduct.

2. Gaseous sulfur dioxide can be passed over an imidazole and the adductforms. The solid adduct may be recovered from the reaction mass bytreating it with a solvent.

3. A solution of an imidazole in a solvent can be added to anotherportion of a similar or the same solvent which is saturated with sulfurdioxide. A precipitate is formed which is the imidazole-sulfur dioxideadduct.

4. Liquid sulfur dioxide can be added to an imidazole (either as thepure compound or as a solution of the compound) and the adduct forms.The mixture is treated with a solvent to recover the solid adduct.

5. A solution of an imidazole can be placed in an autoclave and theautoclave pressurized with sulfur dioxide. The adduct precipitates outof solution.

The sulfur dioxide gas reactions may utilize diluent gases to act ascarriers and controls for the exothermic reaction.

The 2:1 adducts (2 imidazole: 1 SO₂) are more stable than the 1:1adducts. Sulfur dioxide loss occurs with 1:1 adducts at room temperatureover a period of 3 to 4 days. The equation for this decomposition is:##STR3## wherein Imid is as defined previously.

It is difficult to remove the last traces of solvent from the compoundsof Formula (I), and mixture of compounds wherein n has more than onevalue, e.g. 2 and 3, can form.

The adduct-like compounds of this invention decompose or dissociateaccording to the following equation: ##STR4## For a given Imid moiety,the dissociation temperatures of the compounds of formula (I) can varywhen n varies. For example, for 1,2-dimethylimidazole, the dissociationtemperature of the n=2 species is higher than that of the n=1 species.The properties of cured epoxide systems which have been obtained byheating the curable latent systems containing formula (I) compounds canalso vary with n for a given Imid moiety. The dissociation temperaturesare not fixed precisely, but generally appear to be significantly higherthan room temperature. Under normal ambient temperature and pressure,e.g. 23° C./760 mm of Hg, dissociation of the 1:1 (i.e. n=1), 2:1 (n=2),and 3:1 (n=3) adducts according to Equation 2 is, to say the least,difficult to detect. Experimentation with curable epoxide materials suchas diglycidyl ethers of bisphenol A shows that the adducts have little,if any, effect upon the viscosity of the epoxide over a period ofmonths, indicating an epoxide pot life longer than 6 months. This potlife data indicates that little or no free imidazole or imidazolederivative is available for interaction with the vicinal epoxide(oxirane) ring, since free imidazole can cure typical curable epoxidesystems in a manner of minutes at room temperature; see Farkas et al, J.Appl. Polym. Sci. 12, 159 (1968). Even the latentizing the imidazolewith acetic acid to form imidazole acetate salts extends the pot life ofthe epoxide system to only about three weeks; see U.S. Pat. No.3,356,645 (Warren), issued Dec. 5, 1967. Thus, a simple comparison withknown curable epoxide systems provides further evidence that the adductsof the present invention are reasonably stable compounds at normalambient temperature and pressure, at least insofar as loss of theimidazole moiety is concerned. Some clearly detectible loss of imidazolecan occur at temperatures above the melting points of the adducts,particularly in the range of 100° - 200° C., as is subsequently shown bythe data Table II. Since the adducts have varying degrees of stabilityat temperatures above 100° C., the rate or extent of dissociation can beselected in accordance with the desired rate of cure or gel time, as thecase may be. To illustrate: given a temperature of 180° C. and a typicallatent curable system containing a formula (I) compound of thisinvention, the gel time is much longer for Imid = benzimidazole than forImid = imidazole (C₃ N₂ H₄). At 160° C., the benzimidazole adductappears to have little or no effect upon a diglycidyl ether-bisphenol Aepoxide prepolymer; but the imidazole adducts are very effective at thistemperature. The 1-alkyl imidazole species of Formula (I) also exhibitlonger gel times. The properties of the cured epoxides obtainedaccording to this invention are generally satisfactory.

The compounds of Formula (I) are generally solids with fairly smallmelting ranges, typically not more than 5° C. These data can be comparedto the 80°-85° C. melting range of morpholine-sulfur dioxide, which isreported to be a salt-like chemical compound in the U.S. Pat. No.2,270,490 referred to previously. The compound (Imid)₃.sup.. SO₂, whereImid = 2-ethyl-4-methylimidazole, appears to be a viscous liquid, but,as shown subsequently by Table I, this is not typical.

Formulation of curable epoxide systems containing curing agents of thisinvention can be carried out along the lines generally laid down bylatent imidazole epoxide curing technology, e.g. the technologydescribed in the aforementioned 3,553,166 and 3,356,645 patents. TheU.S. Pat. No. 3,553,166 also describes suitable co-curatives (e.g. ofthe dicyandiamide type) which can be included in the curable system. Itis preferred to introduce at least 0.1% by weight, based on the weightof the epoxide monomer or prepolymer, of a compound of Formula (I) intothe curable system, and levels of 1-20% by weight are particularlyuseful. Levels higher than 20% -- even 50% or more -- are permissiblebut excessively increase the cost of the system without any significantbeneficial result.

Typical one-part curable epoxy resin systems formulated according tothis invention comprise (1) 0.1 - 20 parts by weight of a curing agentof Formula (I), (2) at least 80 parts by weight of a suitablecycloaliphatic, aliphatic, aromatic, or heterocyclic epoxide, and (3)0 - 300 parts by weight of suitable fillers, extenders, flexibilizers,pigments, and the like, e.g. colloidal silica. These one-part systemshave sufficient shelf-life at normal ambient temperature to allow formost ordinary shipping and inventory procedures, although the stabilityof the system can be further enhanced, if desired, by specialprecautions such as careful temperature control during storage. It isgenerally not necessary to formulate two-part systems (with the epoxidein one container and the curing agent in another). If a two-part systemis made up, however, the benefits of this invention will still beapparent to industrial users who blend the two parts and then carry outone or more coating, molding, laminating, casting, or impregnating stepsprior to curing at cure temperatures above 100° C. These steps can becarried out in a leisurely fashion, taking advantage of the long roomtemperature pot life prior to curing at elevated temperatures.

Epoxides suitable for use in this invention can be aliphatic,cycloaliphatic, aromatic or heterocyclic and will typically have anaverage epoxy equivalency (i.e. the number of epoxy groups contained inthe average molecule) of from about 1.7 to 6.0, preferably 2 or 3, thisvalue being the average molecular weight of the epoxide divided by theepoxide equivalent weight. The epoxy equivalent weight, which isdetermined by multiplying the sample weight by 16 and dividing by thenumber of grams of oxirane oxygen in the sample, is typically greaterthan 100 for commercially useful curable systems. These materials arevariously referred to as epoxide monomers or prepolymers and in anyevent can contain repeating units, e.g. repeating ether units. Typicalof such epoxides are the glycidyl-type epoxy resins, e.g. the diglycidylethers of polyhydric phenols and of novolak resins, such as described in"Handbook of Epoxy Resins", by Lee and Neville, McGraw-Hill Book Co.,New York (1967).

Another useful class of epoxides has a structure of the following type:

    Ep -- CR.sub.2 -- O--R.sup.1 --O--CR.sub.2 --CROH-CR.sub.2 z O -- R.sup.1 --O--CR.sub.2 -- Ep                                       (III)

or

    R.sup.1 (OCR.sub.2 -- Ep).sub. n                           (IV)

where

Ep is an epoxide ring,

R is hydrogen, or a non-hindering aliphatic group (e.g. methyl);

R¹ is an aliphatic or aromatic radical; and

z is a number from 0 to about 5.

In Formula IV,

n is a number from 1 to 6.

Typically, these epoxides are glycidyl ethers of polyhydric phenolsobtained by reacting a polyhydric phenol or aliphatic polyol with anexcess of chlorohydrin, such as epichlorohydrin, e.g. the diglycidylether of Bisphenol-A or of resorcinol, 1,4-butane diol, or the like.Further examples of epoxides of this type which can be used in thepractice of this invention are described in U.S. Pat. No. 3,018,262(Schroeder), issued Jan. 23, 1962.

The preferred cycloaliphatic epoxide monomers or prepolymers preferablycontain at least one 5- or 6-membered carbocyclic ring (or heterocyclicring with equivalent properties) on which is substituted the epoxidefunctional group. In polycyclic cycloaliphatic epoxides, the two ringsare preferably independent and preferably joined by a bridging radicalcontaining at least one ester or ether linkage. A plurality of theseester or ether linkages can provide flexibilizing properties in thecured system. Further examples of cycloaliphatic epoxide compounds aredescribed in U.S. Pat. No. 3,117,099 (Proops et al), issued Jan. 7,1964.

There are a host of commercially available epoxides which can be used inthis invention, including the diglycidyl ether of Bisphenol-A (e.g."Epon" 828, "EpiRez" 522-C, "Araldite" 7072, "Epon" 1002 and "DER" 332),mixtures of the diglycidyl ether of Bisphenol A with an alkyl glycidylether (e.g. "ERL" 2795), vinylcyclohexene dioxide (e.g. "ERL" - 4206),3,4-epoxycyclohexylmethyl-3, 4-epoxycyclohexane carboxylate (e.g."ERL"-4221),3,4-epoxy-6-methylcyclohexylmethyl-3,4-epoxy-6-methylcyclohexanecarboxylate (e.g. "ERL"-4201),bis(3,4-epoxy-6-methylcyclohexylmethyl)adipate (e.g. "ERL"-4289),bis(2,3-epoxycyclopentyl)ether (e.g. "ERLA"-0400), aliphatic epoxymodified with polypropylene glycol (e.g. "ERLA"-4050 and "ERL"-4052),dipentene dioxide (e.g. "ERL"-4269), epoxidized polybutadiene (e.g."Oxiron" 2001), silicone epoxy (e.g. "Syl-Kem" 90), 1,4-butanedioldiglycidyl ether (e.g. "araldite" RD-2), polyglycidyl ether ofphenolformaldehyde novolak (e.g. "DEN"-431 and "DEN"-438) resorcinoldiglycidyl ether (e.g. Ciba "ERE"-1359), and epoxidized unsaturatedesters of carboxylic acids having more than six carbon atoms, e.g.epoxidized soybean oil. ("Epon" is a trade-mark of Shell Chemical Co.;"EpiRez" is a trademark of Jones-Dabney Co.; "Araldite" is a trademarkof Ciba Products Co.; the various "DER" and "DEN" designations are tradedesignations of Dow Chemical Co.; the "ERL" designations are tradedesignations of Union Carbide Plastics Division; "Syl Kem" is a tradedesignation of Dow Corning; "Oxiron" is a trademark; and "ERE-1359" is atrade designation of Ciba Products Co.)

The compounds of Formula (I) and the nuclei of Formula (II) have alreadybeen described in some detail. As will be apparent from thisdescription, the substituents R¹, R², R⁴, and R⁵ can be variedconsiderably without any adverse effect upon the operability of thisinvention. The teachings of the aforementioned U.S. Pat. Nos. 3,356,645and 3,553,166, and of U.S. Pat. No. 3,361,150 (Green), issued Dec. 28,1971, are generally applicable here with respect to selection ofimidazole substituents, which can also include 5- and 6-member fused orseparate heterocyclic or carbocyclic rings. Substitution at the 1-position (i.e. R¹ percent H) is least preferred. Lower alkylsubstituents (including substituted lower alkyl) are generally mostpreferred, although higher alkyl substituents (containing, for example,7 - 36 carbons) can be used. Other aliphatic (including substitutedaliphatic) radicals can be substituted, as is conventional. Includedamong these are the alkenyl and alkinyl radicals such as allyl. Fusedrings (such as fused benzene or other 6-member carbocyclic rings) arepreferably attached to the 4 and 5 positions; thus R⁴ and R⁵ togethercan comprise the three or four carbons or heterocyclic atoms of a fusedring. Separate aromatic rings can be substituted at the 1-, 2-, 4-, or5- (preferably the 2-, 4-, or 5-) positions and can be monocyclic (e.g.phenyl, tolyl, xylyl, etc.) or polycyclic (preferably di- or tri-cyclic,e.g. naphthyl).

As pointed out previously, the compounds of Formula (I) are generallyuseful when a latent source or other controlled release of either animidazole or sulfur dioxide is needed.

For example: alpha-cyanoacrylate monomers of the formula CH₂ =C(CN)COOR, wherein R can be alkyl, phenyl, etc., polymerize by an ionicmechanism and are sensitive to contaminants, e.g. moisture. Aconsiderable body of patent and scientific literature is availableconcerning the use and storage of these monomers; see U.S. Pat. No.2,776,232 (Shearer et al), issued January 1, 1957, British Pat. No.1,159,548 (Rice et al) published July 30, 1969, U.S. Pat. No. 3,483,870(Coover et al), issued Dec. 16, 1969, and British Pat. No. 1,048,906(Halpern et al), published Nov. 23, 1966. A commercially availableexample of a cyanoacrylate monomer is "Eastman 910", trade designationof Eastman Kodak Company. Sulfur dioxide has conventionally been used tostabilize these monomers. It has now been found that a compound ofFormula (I), preferably one wherein n = 1, can be used as a source ofconstant sulfur dioxide pressure to preserve these monomers in a closedsystem.

The principle and practice of this invention is illustrated by thefollowing non-limiting Examples, wherein all parts are by weight unlessotherwise specified.

EXAMPLE 1

Ten grams of imidazole were placed in a 250 ml. Erlenmeyer flask andexposed to gaseous sulfur dioxide for fifteen minutes. Immediatereaction takes place as is indicated by an exotherm. A light yellowviscous liquid is obtained. An increase in weight of nine gramsresulted. (This liquid will cure epoxy resins immediately and shows nolatency). The viscous liquid is subsequently treated with 150 ml. CHCl₃and stirred rapidly. A fine white precipitate is formed and slowevaporation of the solvent yields 18.5 gms. of product. Drying at roomtemperature under vacuum for three hours yields 16.5 gms. of material.(m.p. = 100° - 104°; 19.9% N; 19.8%S; [N]/[ S] = 2.30.)

The one:one adduct of imidazole and sulfur dioxide is a latent curingagent for epoxy resins. Five parts of the adduct admixed with 95 partsof epoxy resin (Epon 828) shows no increase in viscosity over a periodof months. At 100° C., this mixture has a gel time in excess of 30minutes, whereas a similar mixture of imidazole and epoxy has a gel timeof 5 minutes.

The one:one adduct shows a slow decomposition to the two:one adduct.This decomposition is shown by an almost linear decrease in sampleweight over a period of about 75 hours. There was no further change insample weight from the 80th to the 135th hour, and weighings were thendiscontinued.

EXAMPLE 2

Two moles (136 gm) of imidazole were placed in a three-neck flask fittedwith a condensor, a gas inlet tube, and a stirrer. The imidazole wasdissolved in 200 cc. of chloroform. Gaseous sulfur dioxide (one mole)was bubbled into the solution through the inlet tube as the solution wasstirred. After the addition of the sulfur dioxide, stirring wascontinued for one hour. The solution was transferred to a Rotovapor andthe solvent evaporated off at room temperature. A white solid wasobtained upon removal of the solvent. (M.p. = 70° C; 21.0% N; 12.9% S;[N] /[S] = 3.71.). The elemental analysis indicated that the compound isImid₂ SO₂, where Imid = imidazole. The solid showed no weight loss overa period of months.

Five parts of the solid were added to 100 parts of "Epon" 828 (Trademarkof Shell Chemical Company) resin, which is a diglycidyl ether ofBisphenol A. A portion of this mixture was immediately heated to 160° C.The sample cured to a hard resin in less than three months. The Barcolhardness of the resulting cured sample was 85. Another portion of themixture was stored on the laboratory shelf for 3 months, during whichtime no change in the consistency of the resin was noted. The3-month-old portion was warmed to 160° C., and cured at this temperatureto a hard resin in less than 3 minutes. The Barcol hardness of thiscured resin was 85. (A resin system is considered "cured" when it hasreached the most advanced state of hardening for that system.)

Examples 3 to 11

In Examples 3 to 7, imidazole-sulfur dioxide adducts were prepared inthe same manner as Example 2 from sulfur dioxide and the followingimidazoles: 1-methyl imidazole (Example 3); 2-methyl imidazole (Example4); 1,2-dimethyl imidazole (Example 5); 2-ethyl, 4-methyl imidazole(Example 6); and benzimidazole (Example 7). Data on these compounds oradducts is set forth in Table I. The melting point of morpholine-sulfurdioxide is also given in Table I for comparison.

Examples 8 - 11 are tabulated in Table II and illustrate the use of thecompounds of Examples 1, 2, 5, and 7 with a curable epoxide. The curableepoxide monomer (sometimes referred to as a "prepolymer") is "Epon" 828(trademark of Shell Chemical Corp. for a viscous liquid diglycidyl etherof bisphenol A having an epoxide equivalent weight slightly greater thanthe theoretical 170 and an epoxide functionality of slightly less than2.0). In Examples 8, 9, 10A, and 11A, 95% by weight of the liquidepoxide monomer is combined with 5% by weight of the compound ofExamples 1, 2, 5, and 7, respectively; in Examples 10B and 11B 90 wt. %of the epoxide is combined with 10 wt. % of the compound of Examples 5and 7, respectively. For comparison, data on 5 wt. % imidazole,benzimidazole, and 1,2-dimethyl imidazole are also shown in Table II;the epoxide is again "Epon" 828.

In Table II, the Barcol Hardness test is the standard 935 ASTM Test."Gel time" is a measure of the length of time a sample of resin may bemaintaned in a pliable form at a given temperature.

                                      TABLE I                                     __________________________________________________________________________    Adducts of Imidazoles and Sulfur Dioxide                                                               FOUND         CALCULATED                             Ex.                                                                              Adduct          m.p.  % N % S [N]/[S]                                                                             % N % S                                __________________________________________________________________________        ##STR5##       100-104°                                                                     19.9                                                                              19.8                                                                              2.30  21.2                                                                              24.2                                   ##STR6##       65-70°                                                                       21.0                                                                              12.9                                                                              3.71  28.0                                                                              16.0                                   ##STR7##       60-65°                                                                       20.7                                                                              10.2                                                                              4.73  24.5                                                                              14.0                                   ##STR8##       60-65°                                                                       19.3                                                                              12.5                                                                              3.51  24.5                                                                              14.0                                   ##STR9##       138-140°                                                                     16.7                                                                              15.3                                                                              2.50  17.5                                                                              20.0                                   ##STR10##      Viscous Liquid                                                                      14.5                                                                               5.6                                                                              5.9   19.7                                                                              11.2                                   ##STR11##      135-140°                                                                     18.3                                                                               7.5                                                                              5.58  18.7                                                                              10.7                                   ##STR12##      80-85°                                              __________________________________________________________________________     *U.S. Pat. No. 2,270,490                                                 

                                      TABLE II                                    __________________________________________________________________________    Examples 8 - 11                                                                            Amount    GEL TIME                                                                            (minutes:seconds)                                                                         At:         Barcol Hardness          Example                                                                            Compound of                                                                           (wt. %)                                                                            100° C.                                                                     120° C.                                                                      140° C.                                                                      160° C.                                                                      180° C.                                                                      200° C.                                                                      of Cured                 __________________________________________________________________________                                                         Product                  8    Example 1                                                                             5    30 min:                                                                            9 min:                                                                              2 min:      1 min:      84                                         21 sec.                                                                            19 sec.                                                                             21 sec.                                                                             --    41 sec.                                                                             --                             9    Example 2                                                                             5    41 min:                                                                            21 min:                                                                             4 min:                                                                              2 min:                                                                              1 min:                                                                              --    84                                         5 sec.                                                                             27 sec.                                                                             40 sec.                                                                             10 sec.                                                                             44 sec.                              10A  Example 5                                                                             5    --   --    --    >90 min.                                                                            93 min:                                                                             85 min:                                                                             85                                                                35 sec.                              10B  Example 5                                                                             10   --   --    --    --    17 min:                                                                             2 min:                                                                              85                                                                4 sec.                                                                              13 sec.                        11A  Example 7                                                                             5    --   --    --    >90 min.                                                                            >90 min.                                                                            >90 min.                                                                            80                       11B  Example 7                                                                             10   --   --    --    --    11 min:                                                                             4 min:                                                                              80                                                                30 sec.                                                                             11 sec.                             imidazole                                                                             5    5 min:                                                                             2 min:                                                                              1 min.                                                             3 sec.                                                                             40 sec.     24 sec.                                                                             15 sec.                                                                             --                                  benzimidazole                                                                         5    --   --    55 min:                                                                             2 min:                                                                              1 min:                                                            18 sec.                                                                             40 sec.                                                                             11 sec.                                                                             47 sec;                             1,2-dimethyl                                                                  imidazole                                                                             5    --   --    49 sec.                                                                             41 sec.                                                                             27 sec.                                                                             18 sec.                        __________________________________________________________________________

What is claimed is:
 1. A stabilized composition comprising analpha-cyanoacrylate monomer of the formula CH₂ =C(CN)COOR, wherein R isalkyl or phenyl, admixed with a stabilizing amount of a compound of theformula

    (Imid).sub.n .sup.. SO.sub.2

wherein n is 1, 2, or 3 or mixtures thereof, and Imid is a compound ofthe formula ##SPC1##wherein R¹, R², R⁴ and R⁵ independently representsubstituents selected from the group consisting of hydrogen, analiphatic radical and an aromatic radical, and together R⁴ and R⁵ can bethe residue of a fused ring.
 2. A composition according to claim 1wherein R¹, R², R³ and R⁴ are independently selected from the groupconsisting of hydrogen and lower alkyl of 1 to 6 carbon atoms.
 3. Acomposition according to claim 1 wherein said fused ring is a benzenering.
 4. A composition according to claim 1 wherein R¹, R², R³, R⁴ andR⁵ independently represent substituents selected from the groupconsisting of hydrogen, alkyl of up to 36 carbon atoms, alkenyl,alkinyl; phenyl; tolyl; xylyl and naphthyl; and together R⁴ and R⁵ canbe the residue of a fused benzene ring.